Effect of microstructure on the mechanism of emission for tungsten-barium dispenser cathodes

The effect of tungsten powder fineness and microstructure parameters of the tungsten skeleton on the emission of dispenser cathodes (DC) in the stages of degassing and high-temperature activation are studied. Quantitative analysis of the cathode surface microstructure is investigated. It is established that the work function of the DC after high-temperature activation does not depend upon the size of tungsten powder particles in the test range of fineness (average particle size 1.4-20 mu m) and porosity of the tungsten skeleton (22 and 35%). The time for reaching the maximum DC activity increases with an increase in particle size and skeleton porosity. The highest emission uniformity is exhibited by cathodes with a uniform microstructure prepared from tungsten powder with an average size of 1 and 4 mu m. It is shown that the DC emission capacity is connected with the marked three-dimensional structure of BaO - CaO at the cathode surface, and also monatomic films (Ba - O and Ba) and excess oxygen at the surface of the tungsten phase.